A shadow mask type color picture tube is provided with a shadow mask separated by a certain distance from the luminescent screen in order to selectively guide electron beams emitted from electron guns to the relevant positions on the luminescent screen formed on the inner face of the panel. This shadow mask is made of a thin metal sheet, is provided with numerous apertures, and is weld-fixed to a relatively rigid and heavy frame. The frame is supported by the panel through a proper suspension system or supporting structure.
As described above, the shadow mask is made of a thin metal sheet having countless extremely small apertures. Therefore, the electron beam passing rate is very low, with the result that about 80% of the energy of the electron beams is absorbed into the shadow mask to be converted into heat energy, thereby heating up the shadow mask and the frame. Accordingly, the shadow mask and the frame on which the shadow mask is weld-fixed undergo thermal expansions, with the result that the position of the shadow mask relative to the luminescent screen is displaced, and that the landing positions of the electron beams are deviated, thereby raising the so-called mislanding or purity drift problem.
A conventional shadow mask frame which is proposed in an attempt to overcome such a problem is illustrated in FIG. 1. This device is intended to compensate for the thermal expansion.
In this device, a plurality of hook springs 30 made of bimetals or involving bimetals are attached at a plurality of positions on the four sides of a frame 20 on which a shadow mask 10 is weld-fixed, and the hook springs 30 are respectively coupled with stud pins S installed on the inner circumference of the skirt portion of a panel P on which a luminescent screen L is formed.
The thermal expansion of such a conventional shadow mask frame and the compensation mechanism therefor will be described referring to FIG. 2.
In FIG. 2A, among the electron beams emitted from an electron beam emitting source G such as an electron gun, one electron beam (indicated by an arrow mark of solid line) is emitted in such a manner that it should pass an aperture Ma formed on the shadow mask 10, and should be landed at a point A of the luminescent screen L. Meanwhile, the shadow mask 10 is heated up and thermally expanded by the electron beams. The frame 20, which has relatively larger heat capacity, does not undergo a thermal expansion in the initial stage because heat conduction is not yet sufficient. Therefore, the frame 20 restricts the thermal expansion of the shadow mask 10, which has relatively smaller heat capacity. As a result, the shadow mask 10 is thermally deformed in the form of a dome toward the luminescent screen L. This is called the doming effect.
Accordingly, the aperture which has been at the position MA is displaced to another position MB, and the electron beam travels along the path indicated by a dotted arrow, and lands at another point B of the luminescent screen L, thereby causing a lowering of the color purity. This is called "initial purity drift".
If the color picture tube is continuously operated for a certain period of time, heat is conducted through the shadow mask 10 to the frame 20, and the frame 20 also undergoes thermal expansion. Then, as shown in FIG. 2(B), the frame 20 expands outwardly, while the shadow mask 10 which has been subjected to the doming effect substantially recovers the original curvature which it retained before being heated up. Under this condition, the shadow mask frame assembly thermally expands outwardly, and the aperture which has been at the position MB is displaced to another position MC, with the result that the electron beam travels along the path indicated by an arrow of alternating dashes and dots, and lands at a point C of the luminescent screen L. This is called "long term purity drift".
Thereafter, due to the heat energy conducted to the frame 20, the bimetals or the hook springs made of bimetals (not shown in FIGS. 2(A)-2(C) are heated, and the bimetals or the hook springs push the shadow mask frame assembly toward the luminescent screen L. Therefore, the aperture which has been at the position MC is displaced to another position MA' which is on the same scanning line as the aperture MA, and accordingly, the electron beam travels through the aperture MA' and lands at the point A of the luminescent screen L, which is the originally intended point, thereby achieving a correction for the thermal expansion or the purity drift.
Therefore, during the period of time when one of the apertures of the shadow mask 10 is drifting through the positions MA.fwdarw.MB.fwdarw.MC.fwdarw.MA', that is, during the period of time when the beam landing point is drifting through the points A.fwdarw.B.fwdarw.C.fwdarw.A, the color purity of the images on the screen becomes unstable. The shorter the unstable period, the better quality of images is obtained from a color picture tube.
However, the conventional frame 20 described above has too great a heat capacity compared with the shadow mask 10, and therefore, a long period of time is required until a sufficient thermal expansion and a thermal conduction are attained, with the result that the purity drift period is greatly extended. Further, the supporting structure for compensating the thermal expansion consists of the hook springs 30 which receives the thermal conduction through the side walls of the frame 20, and therefore, the period of time required for the compensation is very much extended. Thus the conventional color picture tube has the disadvantage that it consumes a long period of time before it is stabilized to clear images, i.e., to a sufficient color purity after the starting of its operation.